This invention relates generally to rotary machines and, more particularly, to methods and apparatus for cooling combustion turbine engine components.
Many known combustion turbine engines ignite a fuel-air mixture in a combustor assembly and generate a combustion gas stream that is channeled to a turbine assembly via a hot gas path. Compressed air is channeled to the combustor assembly by a compressor assembly. The output of the turbine assembly may be used to power a machine, for example, an electric generator or a pump.
Airfoils are employed in many known combustion turbine engines, for example, as stationary vanes and rotating blades. Rotating blades are often referred to as buckets. Vanes are typically positioned immediately upstream of associated buckets and may be configured as nozzles. A vane-bucket combination is often referred to as a stage. The buckets are normally coupled to a turbine rotor and the vanes are normally coupled to a stationary portion of the turbine assembly that includes the turbine casing. The combustion gas stream is channeled to predetermined vectors via the vanes such that impingement of the gas stream on the buckets is facilitated. The stages of the turbine assembly facilitate conversion of the thermal energy contained in the combustion gas stream into mechanical energy in the form of engine rotor rotation.
In many known combustion turbine engines, engine efficiency normally increases as combustion gas stream temperature increases. One typical range of combustion gas stream temperatures is approximately 1316° Celsius (C.) to 1427° C. (2400° Fahrenheit (F.) to 2600° F.). In some of these engines, an upper parameter of combustion gas temperature may exist due to the temperature limitations of the materials used to form the affected components. Extended exposure to temperatures exceeding known limitations may induce component deformation or other component life-reducing effects.
Some known methods of attaining desired combustion gas stream temperatures while mitigating the potentially deleterious effects as described above is to introduce a method of cooling the affected components during engine operation. One of these known methods is channeling a portion of an air stream flow from a compressor assembly discharge to the affected components.
In some of the aforementioned known engines, one of the components that may be cooled as described above is the first stage turbine nozzle, sometimes referred to as the S1N (stage one nozzle). The S1N, that includes at least one vane, normally channels the combustion gas stream flow within the hot gas path from the combustor assembly to the set of buckets associated with the first stage of the turbine assembly.
Many known combustion turbine engines channel cooling air to a cavity within the S1N vanes and the air is subsequently channeled to the combustion gas stream via openings in the turbine nozzle vanes, a process often referred to as film cooling. The cooling air stream is typically at a higher pressure than the combustion gas stream, therefore, flow of air into the gas stream is facilitated. Cooler air entering the gas stream via the nozzle vane cavities is disposed to the radially outwardmost section of the nozzle, i.e., the outer surface of the vane, and induces a film cooling effect by forming a layer of cooler air along the outer walls of the vanes, thereby mitigating the effects of the high temperature combustion gas stream on the vanes.
Some known combustion turbine engines that use this form of film cooling of turbine nozzle vanes may induce a reduction of the temperature of the combustion gas stream within the hot gas path prior to combustion gas stream introduction to the first stage buckets of the turbine assembly. The reduction in temperature is due to the cooling air mixing with the higher temperature gas. Some known combustion turbines may experience a gas stream temperature reduction in the range of 80° C. to 150° C. (176° F. to 302° F.). This condition has a tendency to decrease the power output of the turbine assembly for a given rate of combustion, thereby resulting in a decrease in engine efficiency.
One method often used to overcome the decrease in temperature is to increase the firing rate, i.e., the rate of fuel combustion and facilitate an increase in the combustion gas stream prior to the turbine nozzle to restore the temperature of the combustion gas stream at the first stage bucket subsequent to an introduction of cooling air into the gas stream. While the results of this action tends to restore combustion gas stream temperature and the turbine assembly power output, it also increases the rate of combustion.
Increasing the rate of combustion with the subsequent increase in combustion gas temperature above a predetermined threshold value, generally accepted to be approximately 1538° C. (2800° F.), may tend to induce increased formation of nitrogen oxides, often referred to as NOx, i.e., components of combustion gas streams that have a variety of associated environmental issues, including regulatory limitations. To facilitate mitigating a potential for NOx formation, one parameter often observed by engine operators to monitor combustion is a fuel/air ratio, i.e., the ratio of fuel combusted to air used for that combustion. As the ratio decreases, the potential for NOx formation decreases. Generally, combustion turbines operate with lean combustion, i.e., the ratio is as low as practical, with actual ratios in the range of 0.025-0.032. Diverting some of the air discharged from the compressor to the nozzle cooling circuit from the combustion process tends to decrease the air value in the ratio, and the fuel/air ratio tends to increase. As discussed above, these circumstances tend to increase the potential for NOx formation. Hence, it is desirable to minimize the amount of air (discharged from the compressor) that is used for cooling of the nozzles and subsequently discharged into the gas stream. Reducing the predetermined amount of cooling air reduces the performance effects associated with the discharge of the cooling air into the gas stream and lowers the fuel/air ratio, thereby reducing the potential for NOx formation.
The combination of an upper threshold of gas temperatures (due to material limitations and NOx formation) and the narrow range of fuel/air ratios (due to NOx formation) may reduce flexibility in establishing a most efficient mode of operation of a combustion turbine engine.